L. Adhikari

The transport of low-frequency turbulence in the super-Alfvénic solar wind

Understanding the transport of low-frequency turbulence in an expanding magnetized flow is very important in analyzing numerous problems in space physics and astrophysics. Zank et al 2012 developed six general coupled turbulence transport equations, including the Alfven velocity to describe the transport of low-frequency turbulence for any inhomogeneous flows, including sub-Alfvenic coronal flows, and super-Alfvenic solar wind flows. Here, we solve the 1D steady state six coupled turbulence transport equations of Zank et al 2012, and the transport equation corresponding to the solar wind temperature in the super-Alfvenic solar wind flows from 0.29 to 100 AU without the Alfven velocity. We calculate turbulent quantities corresponding to Voyager 2 data sets for three cases; i) a positive and negative sign of Br; ii) the azimuthal angle = tan-1(Bt/Br), and iii) a positive and negative sign of Bt, where Br and Bt are the radial and transverse components of the interplanetary magnetic field, respectively. We compare our theoretical results to the observational results, and find good agreement between them.

The interaction of turbulence with parallel and perpendicular shocks

Interplanetary shocks exist in most astrophysical flows, and modify the properties of the background flow. We apply the Zank et al 2012 six coupled turbulence transport model equations to study the interaction of turbulence with parallel and perpendicular shock waves in the solar wind. We model the 1D structure of a stationary perpendicular or parallel shock wave using a hyperbolic tangent function and the Rankine-Hugoniot conditions. A reduced turbulence transport model (the 4-equation model) is applied to parallel and perpendicular shock waves, and solved using a 4th- order Runge Kutta method. We compare the model results with ACE spacecraft observations. We identify one quasi-parallel and one quasi-perpendicular event in the ACE spacecraft data sets, and compute various turbulent observed values such as the fluctuating magnetic and kinetic energy, the energy in forward and backward propagating modes, the total turbulent energy in the upstream and downstream of the shock. We also calculate the error associated with each turbulent observed value, and fit the observed values by a least square method and use a Fourier series fitting function. We find that the theoretical results are in reasonable agreement with observations. The energy in turbulent fluctuations is enhanced and the correlation length is approximately constant at the shock. Similarly, the normalized cross helicity increases across a perpendicular shock, and decreases across a parallel shock.

Turbulence transport within the Heliosphere

This work continues the investigation of turbulence transport throughout the supersonic solar wind initiated in Zank et al 1996 [27] and Zank et al 2012 [20]. [20] developed a system of six coupled transport equations that describe the transport of energy corresponding to forward propagating (g) and backward propagating modes (f), the residual energy (ED), and the correlation lengths corresponding to forward propagating modes (λ−), backward propagating modes (λ+), and the correlation length (λD) for residual energy. These models can be applied to both sub-Alfvenic (such as the lower corona) and super-Alfvenic (e.g., supersonic solar wind and inner heliosheath) flows. The correlation lengths calculated from our model are in good agreement with those observed. The evolution of related parameters is also calculated from 0.29 AU to 5 AU.